Reduced exercise tolerance is common in cystic fibrosis (CF), yet the effects of a systematic exercise conditioning program in patients with CF have not been determined, despite evidence that such programs increase exercise tolerance in normal people and in adults with chronic obstructive pulmonary disease. The objective of this controlled study is to examine the effects of a formal exercise conditioning program for adolescent and adult CF patients on pulmonary function, oxygen cost of breathing, cardiovascular fitness, physical working capacity, CF clinical score, and patients' attitudes towards themselves, their disease, and their limitations and capabilities. Exercise testing will be done in one of the 5 collaborating exercise laboratories of the National Exercise and Heart Disease Project. Forty patients matched for disease severity will be randomly assigned to control group or exercise group. After exercise, pulmonary, and psychological tests the control group will continue with their usual amount and type of exercise while the exercise group enters a 12-week conditioning program including 3 group sessions weekly of graduated walk-jog endurance work, led by professionals, and adjusted to keep heart rates at 70-85% of each patient's maximum heart rate for 8-10 minutes at the beginning of the study and 20-30 minutes at the end. After 12 weeks, the subjects will be retested and the data analyzed. For the next 12 weeks the control group will undergo the exercise conditioning program, while the exercise group continues to exercise at home. Both groups will then be retested. Four separate 12-week periods will be required to study all of the patients. In the second year, exercise conditioning will be undertaken with another group of patients at home and in local facilities to determine the feasibility and effectiveness of such a program. If exercise conditioning is shown to be beneficial, the implications for thousands of children, adolescents, and adults are far-reaching.